Cree XR-E: how much heatsinking?

[cbr2702]

I just got a new Cree XR-E (R2) that I'm putting into an old bike headlight to run off a dynamo. It's mounted on a star board. Will the metal board be enough to allow the LED to dissipate all the heat it produces? Or do I need to attach a larger heatsink?

Jeff

 

[saabluster]

I just got a new Cree XR-E (R2) that I'm putting into an old bike headlight to run off a dynamo. It's mounted on a star board. Will the metal board be enough to allow the LED to dissipate all the heat it produces? Or do I need to attach a larger heatsink?

Jeff

Depends. How much current is the LED going to see?

 

[cbr2702]

The dynamo is rated for 3W at 6V, which would be 500mA. Except that's ignoring the loss from the bridge rectifier. More problematic, it ignores variance based on speed.

The graph below, from http://pilom.com/BicycleElectronics/DynamoCircuits.htm

Has a "circut 2" (orange line) almost identical to what I have. (I don't have a smoothing capacitor on it right now, so it flickers at low speeds).

So. I think I should never be running it above 500mA.

 

[JohnR66]

There is no easy answer because one has to know how the heat will be removed from the heatsink. A heatsink bolted to a metal enclosure can be smaller than one inside a plastic enclosure.

A 2in sq plate is probably plenty for it at this current.

 

[Oznog]

A heatsink sealed inside the case doesn't do a lot of good either. Well, it does some but really you either need to let air into the case- which defeats the waterproofing purpose of the case- or an external sink.

Or you could put the electronics in a separate box- or inside the dynamo housing- and mount the LED on a strip of aluminum with an acrylic cover. That's kinda kickass because the LED head is really "minimal".

The Star board, in free still air, is probably "good" for ~1W. Temp may be on the high side inside an enclosure at even 1W. Actually even at 1W most designers would give it a heatsink (or connect it with an aluminum case like the flashlights do) if it were going into a quality product.

 

[cbr2702]

Well, if the board is good for maybe 1W in free still air and I'm running it closer to 2W in an enclosure, that sounds bad.

The led is not bolted to anything; for the time being it's electrical taped inside the housing where the bulb used to be.

When I went out for 20min yesterday the star was cool to the touch, but then again it was 15F out.

What if I put a small fan (like a computer case fan) inside the housing? Pretty sure it would only draw about 10mA. One problem would be finding one that worked at the lower voltage (all the ones I have want 12V) but that shouldn't be too bad. Or would moving the air inside the housing not help much?

I could run a strip of metal out the edge of the light housing to cool in the breeze and attach that to the star. Any guesses on how thick the metal would have to be to be effective? I know people usually attach with thermal glue, but would solder work just as well?

 

[saabluster]

What you are trying to do although doable is not reasonable. Unless it is just for the fun of it. I would just buy a proper LED flashlight and mount it to your bike with one of those universal mounts. You could still rewire it to run off the dynamo if you wanted to but at least you would have a good foundation to build from.

Well, if the board is good for maybe 1W in free still air and I'm running it closer to 2W in an enclosure, that sounds bad.

The led is not bolted to anything; for the time being it's electrical taped inside the housing where the bulb used to be.

When I went out for 20min yesterday the star was cool to the touch, but then again it was 15F out.

What if I put a small fan (like a computer case fan) inside the housing? Pretty sure it would only draw about 10mA. One problem would be finding one that worked at the lower voltage (all the ones I have want 12V) but that shouldn't be too bad. Or would moving the air inside the housing not help much?

I could run a strip of metal out the edge of the light housing to cool in the breeze and attach that to the star. Any guesses on how thick the metal would have to be to be effective? I know people usually attach with thermal glue, but would solder work just as well?

 

[cbr2702]

The "fun of it" is an aspect. I'm replacing the innards of an old bike light to go with an old style bike. If I have to I'll do something else, but I'd like to get this to work.

I'm thinking more about just running a good sized strip of metal out the bottom of the light to cool in the breeze. Certainly better than a fan if it will work. The only thing I'm not sure about with this, though, is whether soldering the heatsink to the star will conduct heat well or if I need to go buy some arctic silver or the like.

 

[cbr2702]

I ended up soldering a thick copper wire (20A house wiring ground cable) to the star. If it feels hot when I'm riding tomorrow I might solder the other end to the metal housing as well.

It's sort of ugly, but all the ugly is inside the housing. From the outside it looks like an old raleigh headlamp with crazy amounts of light.

 

[yellow]

2 Problems:
that series connected MC-E (it is series, right?) will need a bit of speed to light up.
Three led in series is perfect with Dynamo, 4 could be a tiny bit too much.

heatsinking is NEEDED!!!!
If that housing You use is from metal, then mount the Star onto it with thermal paste, then fix with screws or epoxy glue around the star.
That way the housing acts as heatsink and You save weight and size of a (totally useless) sink inside the housing.
If from plastic, mount the star onto a 2-at-least mm thick sheet of aluminium, as wide as the star, and have it extend outside of the housing (maybe bent along the outside), a few cm long.
Or use it to mount the whole light to the bike.

When the heatsinking metal You use now does not get warm, it is else much to small, or Your dynamo still has power restriction inside (= antiparallel Z-Diodes). To try that, disconnect any wire from dynamo and hook a voltmeter to it (AC). If the "no-load" Voltage does not get over 7.5V, You have to get rid of the Diodes before the MC-E works fine. (Value shown should be in the 20 Volts)

I can type this, because I have a very quick and easy mod with cheap bike headlights done often. It uses a Lux V (which I still have for that) that is mounted on ~0.5" Alu-stick that is about finger-long and extends at the back end of the plastic housing. After a few mins of riding, when toched there You can almost burn Your finger.
And this Lux V should run way cooler, as it basically is just half of Your setup

(my setup is a triple cree light)

 

[evilc66]

Yellow, it's an XR-E R2, not and MC-E.

 

[yellow]

damn, I am so much in my MC-E project, I read that phrase everywhere ....

XR-E, yes?
Then use at least two in series,
if Your dynamo is not voltage restricted (= considerably over 7.5 V, measured with no load), make that a three

 

[cbr2702]

The brightness is not an issue. I'm quite happy with the light output of even one XR-E R2 (and I should probably stop reading this forum if I want to remain satisfied with my current light performance).

On my way home from work monday I stopped maybe 20min to see how hot the thing had gotten. The copper wire felt noticeably warm, but not hot. I think I'm going to try soldering the wire to the housing (which is thin chromed steel).

The main issue is that soldering and heatsinking don't go well together. You want to get the soldered bit really hot while keeping the rest of it from getting hot enough to release the bits you've already soldered on.

I know this is hackish, but is there any reason using a powerful (300W) soldering gun on one end of the wire, soldering it to the case, while using ice at the other end of the copper to keep the led cool, wouldn't work? I think it would be annoying, and I'd need to not let things get wet, but should work.

 

[JWP_EE]

You could grip the wire with a pair of pliers. This will act as a heat sink and keep most of the heat from traveling back down the wire.

 

[cbr2702]

I finished adding an (ugly) breeze catching mass of copper. It's completely cold to the touch now. So unless I borked the thermal connection to the led's star, it's working great.

http://www.bikeforums.net/showthread.php?t=467186

 

[Oznog]

I ended up soldering a thick copper wire (20A house wiring ground cable) to the star. If it feels hot when I'm riding tomorrow I might solder the other end to the metal housing as well.

It's sort of ugly, but all the ugly is inside the housing. From the outside it looks like an old raleigh headlamp with crazy amounts of light.

Isn't the Star aluminum??
Soldering is impossible except with some exotic processes. You can sometimes get solder to appear to flow onto aluminum but it's fake there's no real bond. Mechanically it will simply fall off at the first opportunity.

You wanna screw a Star onto an aluminum heatsink. For 3W, something like 3 or 4 total sq in of exposed, unpainted aluminum surface is probably a good minimum. A sink can be much smaller if finned because it'll get the same surface area or better in a much smaller package.

If it's inside a case then there's a real question of how that heat's getting out.

 

[cbr2702]

Ok. I admit to more kludgery. The star is aluminum. So no real soldering possible. The 12 gauge copper wire is held onto the star with tightly wrapped thin copper wire. Then molten solder was dripped onto the mass of copper and aluminum to maximize the amount of contact. The bond needs to hold very little force, so I'm not too worried.